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Appendix VL

1125

means a worm gear that will overhaul. If the worm is to hold
the wheel we refer to the lower end of the right-hand curves.
The points where these intersect the base line indicate the angles
of friction, and consequently the angle of thread for a retaining
worm.

P. 8%. Efficiency of Screw.—The apparatus required
for the experiment described on p. 874 is represented in Fig. 994.
A common 2-ton screw-jack A is mounted on a baseboard c,
being further supplied with a pulley D, a bolt B to hold the
weights, and a plate L to support them: the, last-named being

or Scr&us

uy. 994.

j__________            i

6         'MS          O

K

\\.f

securely fastened to the screw-head. A string from D passes over
a pulley E, running on bicycle ball-bearings j, and carries the
small weights G that are equivalent to the effort The bracket K
is held by one central bolt F so^that it may be swivelled in plan
to allow the cord to lead off tangentiatty to f> for the two expert
meats of raising and lowering. Tfee friction of the ball bearing is
inappreciable, md can be neglected altogether, so that if G be